A suicide bomber attacked Pakistani soldiers escorting census workers on the outskirts of the eastern city of Lahore on Wednesday, killing at least five army personnel and two civilians, officials said.
The bomber targeted a van carrying 17 soldiers, among the 200,000 deployed nationwide to provide security for Pakistan's first census in nearly two decades.
Nearly 20 people were injured, officials said.
It was the latest deadly attack to strike Lahore, which had long been insulated from Pakistan's endless drumbeat of extremist violence but this year has seen a series of major bomb blasts.
Malik Ahmed Khan, spokesman for the Punjab provincial government, said officials had received information last week about a threat against census takers, but the intelligence did not specify the target.
Pakistan's census commissioner Asif Bajwa said that security arrangements would be reviewed but the counting process would continue.
"Such cowardly acts cannot stop us," Bajwa said, adding that additional security teams had already been dispatched to Lahore.
The army chief, Gen. Qamar Javed Bajwa, condemned the attack but said the census "will be completed at any cost."
Police responding to the blast, which initial reports said involved at least 15 pounds of explosives, found a grisly scene. The severed head of the bomber, who appeared to be in his early 20s, landed on the roof of a 40-foot-tall building, according to a police official who requested anonymity because they were not authorized to speak to the media.
The official said the driver of the van carrying 17 army personnel had stopped due to an apparent breakdown and was showing the vehicle to a mechanic when the blast occurred. The driver, who was injured, was also being questioned by police.
As they often do, Pakistani officials blamed neighboring Afghanistan for the violence, saying militants used sanctuaries there to plot attacks.
"Most of the recent attacks in Lahore were carried out by suicide bombers trained in Afghanistan," said Rana Sanullah, law minister of Punjab province.
Afghanistan counters that Pakistan must crack down on extremist sanctuaries on its own soil, including those where leaders of the Taliban and other groups are based.
The census is a highly charged political issue in Pakistan, which hasn't undertaken a complete count of its population in 19 years. Before counting began in February, ethnic minority groups and residents of poorer provinces expressed worries that they would be undercounted due to insecurity.
Instead, the attack on Wednesday struck Punjab, Pakistan's richest and most populous province. It was the third major militant attack in Lahore, the provincial capital, in two months, following bombings in February at a protest rally and a busy market.
Special correspondent Sahi reported from Islamabad and Times staff writer Bengali from Mumbai, India.
Follow @SBengali for more news from South Asia
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7 a.m.: This article was updated throughout with staff reporting and a higher death toll.